1. Field of the Invention
This invention relates to thin layer optical interference coatings for reflecting infrared radiation and transmitting visible light radiation and their use on lamps. More particularly, this invention relates to optical interference coatings made of alternating high and low refractive index layers for reflecting infrared radiation and transmitting visible light radiation which comprise three spectrally adjacent multiperiod stacks with the first stack being a conventional short wave pass stack having at least two periods and with the second and third stacks each comprising at least two periods and being spectrally located at increasingly longer wavelengths than the first stack, and lamps containing such filters.
2. Background of the Disclosure
Thin film optical interference coatings known as interference filters which comprise alternating layers of two or more materials of different refractive index are well known to those skilled in the art. Such coatings or films are used to selectively reflect and/or transmit light radiation from various portions of the electromagnetic spectrum such as ultraviolet, visible and infrared radiation. These films or coatings are used in the lamp industry to coat reflectors and lamp envelopes. One application in which these thin film optical coatings have been found to be useful is to improve the illumination efficiency or efficacy of incandescent and arc lamps by reflecting infrared radiation emitted by a filament or arc back to the filament or arc while transmitting the visible light portion of the electromagnetic spectrum emitted by the filament or arc. This lowers the amount of electrical energy required to be supplied to the filament or arc to maintain its operating temperature. In other lamp applications where it is desired to transmit infrared radiation, such filters can reflect the shorter wavelength portions of the spectrum, such as ultraviolet and visible light portions emitted by a filament or arc and transmit primarily the infrared portion in order to provide heat radiation with little or no visible light radiation. Such an application of this latter type would include a typical radiant heater for residential or industrial use where visible radiation emitted by the heater is unwanted.
Optical interference coatings or filters used for applications where the filter will be exposed to high temperatures in excess of 500.degree. C. or so have been made of alternating layers of refractory metal oxides such as tantala (tantalum pentoxide Ta.sub.2 O.sub.5), titania (titanium dioxide TiO.sub.2), niobia (niobium pentoxide Nb.sub.2 O.sub.5) and silica (SiO.sub.2), wherein the silica is the low refractive index material and the tantala, titania or niobia is the high refractive index material. Such filters and lamps employing same are disclosed, for example, in U.S. Pat. Nos. 4,588,923; 4,663,557 4,689,519 and 4,734,614. In such lamp applications, the filters are applied on the outside surface of the vitreous lamp envelope containing the filament or arc within and often reach operating temperatures as high as 900 C. These interference filters or coatings are applied using evaporation or sputtering techniques and also by chemical vapor deposition (CVD) and low pressure chemical vapor deposition (LPCVD) processes. Some attempts to make such interference filters have employed solution deposition techniques such as is disclosed in U.S. Pat. No. 4,701,663. Solution deposition techniques however produce relatively thick layers which tend to crack and which severely limits the filter design. CVD and particularly LPCVD deposition processes are preferred for applying coatings to other than flat objects such as reflectors and lamps.